Maximum link range of a mobile device operating in a mobile communication network is strongly dependent on the maximum available output power of such device. Typically, the maximum available output power of mobile device has an effect on the base station density in the network and also on the network's operating expenses and capital expenses.
The maximum output power of a mobile device is dependent on the maximum available DC power, signal waveforms, and minimum possible link distance to adjacent base stations operating on adjacent channels. The minimum distance to the base stations operating on adjacent channels is important because power amplifiers (PA) are non-linear elements and thereby generate so-called adjacent channel power (ACP). RF (Radio Frequency) specifications of systems typically set restrictions to the power that is radiated on the adjacent channels. The reason for setting such restrictions is that ACP interferes with reception of the adjacent base stations if the ACP level is too high.
The restrictions of the specifications are conventionally derived on the basis of the minimum attenuation from the mobile device to the adjacent base stations and on the basis of integrated noise+noise figure over the channel bandwidth. That is, the restrictions are derived on the basis of the worst-case scenario.
Especially in systems, which use signals with high PAPR (Peak-to-Average-Power Ratio), ACPR (Adjacent Channel Power Ratio) requirements of specifications restrict the maximum transmission power level of the mobile devices and PAs of the mobile devices have to continuously operate with high backoff and with very small efficiency in order to maintain sufficient linearity. For example, in OFDM (Orthogonal Frequency Division Multiplexing) PA backoff can be almost 8 dB.
PA backoff is defined as follows: backoff is the ratio of power that is transmitted from the PA to the −1 dB compression point of the PA in dBs.
Because the ACPR requirements are derived on the basis of the worst-case scenario, the whole system is actually operating with low power efficiency and the idle and operation times or ranges of the system remain small.
Typically the upper bound of the output power is determined so that it is possible to implement required TX (transmission) spectrum mask with reasonable power consumption. If the requested output power is less than the available maximum, it is possible to save DC power by tuning for example the supply voltage with DC-DC converters and reducing the output power this way. Current specifications do not allow higher values of ACP than the value derived from the minimum attenuation to receivers operating on adjacent channels. Typically, power control is designed to take into account only in-band power or SINR (Signal-to-Interference and Noise Ratio) requirements.
PAPR reduction techniques can be used for reducing the power amplifier backoff. A technique that is called narrow band or soft clipping reduces probability of high amplitude peaks and in this way reduces the PA backoff one to two dBs. It is possible to clip the signal even more if the required signal quality is not high.
In addition to these measures, there is a need to develop more efficient power control algorithms.